The invention relates to a method for the production of an electrically conductive wall of a fabric and a film material, which is provided with an insulating coating, and particularly for the production of sheets of fabrics, which are provided with an insulating coating and have electrically conductive tapes, these materials being used particularly for the production of flexible containers for bulk goods.
Flexible bulk goods containers are used for packing, transporting, enveloping and storing of different bulk goods material in amounts, in each case of 500 to 2,000 kg. They are also used in areas where there are increased dangers of explosion due to an explosive atmosphere of gases, vapors or mist and due to the combustibility of the bulk goods themselves. An explosion could be initiated by electrostatic charges on the walls of the bulk goods container, such as those charges, which arise when electrically insulating bulk materials are filled into or emptied from an electrically insulated container. Bulk goods containers, which can discharge electrostatic charges, must therefore be used in all areas, in which the danger of an explosion exists due to gases, vapors or dusts.
In designing bulk goods containers, which are cable of discharging electrostatic charges, basically two types of electrostatic charging and discharging phenomena must be taken into consideration.
To begin with, brush discharges, which emanate from charged surfaces of insulating materials and are responsible for the risks in the case of gases and vapors with a low minimum ignition energy, must be taken into consideration. On the other hand, sliding brush discharges, which occur at charge double layers, are important, because appreciable energies are converted by such discharges and thus also have the potential for igniting dusts. Brush discharges can be prevented if the charged surfaces are not larger than 10.sup.4 mm.sup.2. Sliding brush discharges then no longer take place when the breakdown voltage of the double layer is less than 4 kV.
Usually, flexible bulk goods containers are produced from flexible polypropylene tapes. By weaving electrostatically conductive tapes in the warp and filling (weft) directions, the basic container fabric is divided into electrostatically separated area elements, which are not larger than 10.sup.4 mm.sup.2, so that the brush discharges, which emanate from the charged surfaces of insulating materials, do not arise.
Since flexible bulk goods containers of fabric tapes basically are not dust tight, they are provided on their inside with a coating, preferably of polypropylene. In order to prevent sliding brush discharges, which can arise due to the high charge potential at the coating, the breakdown voltage must not exceed 4 kV. This is achieved, by a layer, the thickness of which does not exceed 30 .mu.m. The fabric below can be disregarded for the breakdown resistance, since the fabric of polypropylene tapes, as a result of its porosity, does not have a breakdown resistance higher than that of air.
However, due to the coating, contact with the electrically conductive tapes is prevented, so that the danger of the occurrence of brush discharges, capable of igniting gases and vapors, remains.